The present invention relates to an automatic sound volume control device. More particularly, the invention relates to an automatic sound volume control device which allows a listener to clearly hear reproduced sound at places where the ambient noise level is high.
When music is played in a high ambient noise environment, the music may be masked by the noise. For instance, when music is being played by a stereophonic tape player or radio in a moving automobile, the music may be intermittently masked by the ambient noise as the noise level varies or the music sound level changes.
In order to solve this problem, an automatic sound volume control device which adjusts the output sound level in response to a detected ambient noise level has been proposed. In an example of such a device shown in FIG. 1, the ambient noise is picked up and converted into an electrical signal by a detector 1. This electrical signal is supplied to a noise level detecting circuit 2 which produces a signal representing the level of the noise. The noise level signal thus produced is multiplied by a constant .alpha. in a scale factor multiplier circuit 4, which resultingly outputs a gain control signal A.
A desired voice or music signal B applied to a signal input terminal 3 is supplied through a gain control circuit 5 to a power amplifier 6 where it is amplified. The amplified signal is reproduced by a loudspeaker 7. In this operation, the gain control circuit 5 varies the gain in accordance with the gain control signal A outputted by the constant circuit 4. That is, as the ambient noise level increases, the level of the sound reproduced by the loudspeaker 7 is increased so that the listener can easily hear the desired reproduced sound. The relationship between the ambient noise level and sound volume is shown in FIG. 2, from which it can be seen that the average sound volume b is increased as the ambient noise a increases.
In the above-described device, the level of output sound produced is varied according only to the level of the ambient noise. Therefore, the device still suffers from a drawback in that it is not so effective in a case when the input (desired) signal level varies significantly.
For example, for the case of a symphony which has a considerable variation in average sound level, a low level music sound is masked in a range D as shown in FIG. 3A. If the gain is increased in order to prevent the low level signal c from being masked, then a difficulty occurs in that the high level signal e is excessive in sound volume at a range f as shown in FIG. 3B, possibly causing distortion of the signal in the loudspeaker or power amplifier.
Accordingly, an object of the invention is to produce an automatic sound volume control device which is effective even when the input signal level varies.